1. Field of the Invention
The present invention relates to a single-chip microcomputer, and more specifically to a single-chip microcomputer including therein an erasable programmable read only memory (called "EPROM" in the specification) which can be written, read and erased.
2. Description of related art
In general, a single-chip microcomputer has included a central processing unit for executing data processing, a memory section for storing programs and data, and an input/output section for performing a data transfer between the microcomputer and an external device, the central processing unit, the memory section and the input/output section being assembled on a single integrated circuit chip. This type of microcomputer, which is one of stored program controlled apparatuses, can execute different processings by changing a program, and therefore, has been widely used as a control device in various fields.
However, a program memory and a data memory have to have different capacities required dependently upon application systems and processings to be executed. Therefore, different microcomputers which have a central processing unit and an input/output section of the same functions but which include a different capacity memory, have been now developed and manufactured.
On the other hand, with a recent developed integrated circuit technology, single-chip microcomputers including an EPROM as a program memory have appeared, in addition to those which include a read only memory having a program permanently built in during its manufacture in accordance with a specially created pattern or mask (called "mask ROM" hereinafter).
The single-chip microcomputer including the EPROM therein is advantageous in that even after the single-chip microcomputer is assembled in a package and put in a finished condition as an integrated circuit, a program can be written to the EPROM of the single-chip microcomputer. Therefore, since the single-chip microcomputer can shorten the time required from a design of a program until actual operation of the system, the single-chip microcomputer has been widely utilized for program evaluation in the course of system development and for small-amount production.
In the case of utilizing the single-chip microcomputer including the EPROM therein for the purpose of evaluating a program to be used in an apparatus including a mask ROM therein, in some cases a completely equivalent evaluation cannot be executed because of differences in memory capacity between the EPROM and the mask ROM.
For example, in a system having a function of discriminating a space of a program memory and fetching an instruction from an external memory if an accessed memory space is not within the memory included in the single-chip microcomputer, if the memory capacity is different between the microcomputer having the EPROM and the microcomputer having the mask ROM, a boundary between the internal memory and the external memory in the microcomputer having the EPROM is also different from the internal/external memory boundary in the microcomputer having the mask ROM, and therefore, a memory space from which an instruction is read is also different. In addition, if an internal data memory has a different capacity, a memory access to a memory space corresponding to a memory having a small memory capacity can be equivalently executed, but data stored in a memory space having a large memory capacity becomes different. Accordingly, in order to evaluate a program by using a system having a different memory capacity, it is necessary to design the program taking the difference in consideration, and to check whether or not there is access to a memory space which does not actually exist. Because of the above mentioned circumstance, a device including an EPROM, completely equivalent to a device including a mask ROM, has been required.
At present, however, the amount of use of devices including an EPROM is one-tenth or less of the amount of use of devices including a mask ROM. Therefore, it would be very expensive and not so economic to develop and manufacture a system including therein a number of EPROMs corresponding to different memory capacities.